Spatial division multiple access (SDMA) is a technique for the effective use of finite frequency resources in a wireless communication system. The SDMA is a method of allowing a plurality of user equipments (UEs) to simultaneously access a base station (BS) by using a spatial channel property of UEs that can be spatially divided. The SDMA can be used for system performance improvement in a system in which the BS employs a plurality of antennas and each EU employs at least one antenna.
Examples of the conventional method for implementing the SDMA will be described below.
1) In one method, each UE transmits a preamble before data transmission and the BS estimates a channel of each UE by using the received preamble, and then the BS implements the SDMA by performing beamforming so that as many messages as possible can be received when several UEs performs channel access according to an estimated channel state.
2) In another method, under the assumption that the BS knows channel information of UEs desiring channel access, a signal to interference and noise ratio (SINR) is calculated for a case where all or some of UEs access the channel in one time slot, and the UEs are allocated to the time slot so that an SINR of each time slot satisfies a threshold.
3) In another method, UEs capable of simultaneously receiving services are grouped according to a metric determined by the BS under the assumption that the BS entirely knows channel information of each UE.
In the above description, the method 1) is inferior in performance to the method of controlling channel access by grouping the UEs capable of performing channel access.
In the method 2) and the method 3), in order to know channel information of each UE, the BS has to receive a channel request message as well as information indicating whether each UE desires channel access. Further, after grouping the UEs capable of performing simultaneous channel access, the BS has to report a group of each UE through a downlink to each UE. Such a process requires a lot of resources. In addition, as the number of UEs increases, a computational complexity also increases in the grouping of the UEs, which results in the increase of time delay.